Actually CSS3 introduces the "calc()" functional notation, which may allow you do such a calculation. Safari 5.x and Opera don't support it, but other major browsers do, including Safari 6 and IE9 and newer (if you use the "-webkit-" and "-moz-" prefixed variants). IE5-7 support "expression()" which provides a similar ability, however, note that support for neither "calc()" nor "expression()" is available in IE8's standards mode.